The measurement of consistency is an important operation of any mechanical pulp, yet no reasonably reliable, relatively quick test an available for determining consistency of pulp of higher consistencies.
In a mechanical pulp mill, pulp is produced by working (refining) wood chips or the like between a pair of opposed plates in a conventional disc type refiner. This pulping operation is carried out at a relatively high pulp consistency (generally in the range of 25-50%). Obviously it is important that the plates, which are biased toward one another by the pressure load on the refiner during operation not clash as this will damage them. There are three main variables which effect the plate spacing namely, the setting of disc spacing, the specific energy applied to the pulp, and the consistency of the pulp. Generally the discs are first set at a zero spacing where the plates do not clash under no load conditions. The load is applied by feeding pulp between the discs. This load actually deflects the discs and moves them farther apart towards their peripheries which permits the disc setting to be reduced, i.e. the discs moved closer together so that the discs spacing at full load operation would result in the discs clashing together if there were no pulp between the discs. The refiners are generally operated at full motor load for a pre-set feed rate and to obtain a targeted specific energy application to the pulp. To apply the targeted specific energy the disc spacing is set as required. The consistency of this pulp has a significant effect on the pressure between the plates, i.e. the forces tending to hold the plates apart. If the consistency is reduced significantly the pressure between the plates will reduce and may cause the discs to clash and damage the refiner. It is thus a major advantage to know the consistency of pulp leaving the refiner, but there is no adequate means for measuring the moisture content of the wood entering the system or the amount of moisture flashed in the refiner and leaving the system as steam both of which contribute significantly to consistency. Thus no reliable method of measuring the in-coming wood is available that will give a clear indication of consistency.
It is well known to measure the consistency of the pulp leaving the refiner however such measurement cannot be accomplished rapidly and provide reliable results at the consistencies normally found in refining.
One of the standard methods of measuring consistency currently used in mills requires obtaining a sample, weighing the sample, driving off the moisture, and weighing the residual to obtain a ratio of liquid to bone-dry solids in the pulp. Such a procedure obviously is time consuming. Modified versions are used in the mills, but due to the elaborate procedures and normal mill working environments, such procedures have been found to be relatively inaccurate.
An experienced operator can estimate quite accurately the consistency of a pulp sample in the range of 15-50% by sensing from the pressure his hand must exert to squeeze out water therefrom. However, such judgement is obviously subjective and can only be developed by long experience in the mill and thus is not a satisfactory control test for mill operation unless the operator has had the necessary experience.
Consistency measurement of chemical pulp of high consistency is not as prevelent as with mechanical pulps but there are applications where such measurement is required.